Messung der semileptonischen $\Xi^{0}$-Zerfälle mit dem NA48/1-Detektor

In 2002 a high intensity data acquisition for K_S mesons and neutral hyperons was performed by the NA48/1 experiment, in which about 10^9 Xi^0 decay candidates were recorded. Within this work 6657 Xi^0 -> Sigma^+ e^- anti-nu and 581 anti-Xi^0 -> anti-Sigma^+ e^+ nu events were selected from that data sample and the branching ratios BR1(Gamma(Xi^0 -> Sigma^+ e^- anti-nu)/Gamma(Xi^0 total))=( 2.533 +-0.032(stat) -0.076+0.089(syst) )10^-4 and BR2(Gamma(anti-Xi^0 -> anti-Sigma^+ e^+ nu)/Gamma(Anti-Xi^0 total))= ( 2.57 +-0.12(stat) -0.09+0.10(syst) )10^-4 were determined. The result for BR1 is 3.5 times more precise than the previously published measurement. The analysis of anti-Xi^0-beta decays is the first measurement of BR2. Both results agree with the theoretical prediction of 2.6*10^-4. From BR1, the CKM matrix element |Vus| = 0.209 +- 0.004(exp) +- 0.026(syst) was determined by using the experimental value for the form factor ratio g1/f1. The dominant uncertainty is given by the error on g1/f1. Besides, 99 Xi^0 -> Sigma^+ mu^- Anti-nu decay candidates including an estimated background of 30 events were reconstructed within this work. The measured branching fraction is BR3(Gamma(Xi^0 -> Sigma^+ mu^- anti-nu)/Gamma(Xi^0 total)) = ( 2.11 +- 0.31(stat) +- 0.15(syst) )10^-6